Astronomers have mistakenly reported observations of a double star in place of J 900 and a faint star in the constellation of Gemini.
Binary system of two stars
Detail of Bayer's chart for Orion showing the belt stars and Orion Nebula region, with both Greek and Latin letter labels visible
Artist's impression of the discs around the young stars HK Tauri A and B.
Edge-on disc of gas and dust present around the binary star system HD 106906
Orion constellation map
Algol B orbits Algol A. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase.
Artist's conception of a cataclysmic variable system
Artist's impression of the binary star system AR Scorpii
Artist rendering of plasma ejections from V Hydrae
Artist's impression of the sight from a (hypothetical) moon of planet HD 188753 Ab (upper left), which orbits a triple star system. The brightest companion is just below the horizon.
Schematic of a binary star system with one planet on an S-type orbit and one on a P-type orbit
The two visibly distinguishable components of Albireo
Luhman 16, the third closest star system, contains two brown dwarfs.
Planet Lost in the Glare of Binary Stars (illustration)

This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer.

- Double star

The more general term double star is used for pairs of stars which are seen to be close together in the sky.

- Binary star

Some bright visual double stars have a Bayer designation.

- Double star

Usually these are double stars (mostly optical doubles rather than true binary stars), but there are some exceptions such as the chain of stars π1, π2, π3, π4, π5 and π6 Orionis.

- Bayer designation

In cases where the binary star has a Bayer designation and is widely separated, it is possible that the members of the pair will be designated with superscripts; an example is Zeta Reticuli, whose components are ζ1 Reticuli and ζ2 Reticuli.

- Binary star
Astronomers have mistakenly reported observations of a double star in place of J 900 and a faint star in the constellation of Gemini.

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A visual band light curve for YY Geminorum (Castor C), adapted from Butler et al. (2015)

Castor (star)

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Second-brightest object in the zodiac constellation of Gemini.

Second-brightest object in the zodiac constellation of Gemini.

A visual band light curve for YY Geminorum (Castor C), adapted from Butler et al. (2015)

It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem.

Castor appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs.

Appearing to the naked eye as a single star, Castor was first recorded as a double star in 1718 by James Pound, but it may have been resolved into at least two sources of light by Cassini as early as 1678.